Xcode 12 warning for FlexibleDiff

Building FlexibleDiff on Xcode 12 gives the warning:

`The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 8.0, but the range of supported deployment target versions is 9.0 to 14.0.99.`

Updating the `deployment_target` in the podspec will remove this issue, but will also drop support for iOS 8 devices.